LSI 53C875A - Manual (Page 172)

4-80 Registers
check byte are received from the SCSI bus (all signals
are shown active HIGH):
A one in any bit position of the final SLPAR value would
indicate a transmission error.
The SLPAR register is also used to generate the check
bytes for SCSI send operations. If the SLPAR register
contains all zeros prior to sending a block move, it
contains the appropriate check byte at the end of the
block move. This byte must then be sent across the SCSI
bus.
Note: Writing any value to this register clears it to zero.
The longitudinal parity checks are meant to provide an
added measure of SCSI data integrity and are entirely
optional. This register does not latch SCSI
selection/reselection IDs under any circumstances. The
default value of this register is zero.
The longitudinal parity function normally operates as a
byte function. During 16-bit transfers, the high and low
bytes are XORed together and then XORed into the
current longitudinal parity value. By setting the SLPMD bit
in the
SCSI Control Two (SCNTL2) register, the
longitudinal parity function is made to operate as a
word-wide function. During 16-bit transfers, the high byte
of the SCSI bus is XORed with the high byte of the
current longitudinal parity value, and the low byte of the
SCSI bus is XORed with the low byte of the current
longitudinal parity value. In this mode, the 16-bit
longitudinal parity value is accessed a byte at a time
through the
SCSI Longitudinal Parity (SLPAR) register.
Data Bytes Running SLPAR
– 00000000
1. 11001100 11001100 (XOR of word 1)
2. 01010101 10011001 (XOR of word 1 and 2)
3. 00001111 10010110 (XOR of word 1, 2 and 3) Even Parity
4. 10010110 00000000